The present invention relates to a method of determined fatigue life of metals, and specifically to a method of determining fatigue life by irradiating a metal that is subject to receive cyclic load with X-rays and by using a half-value width of a curve of diffracted X-ray intensity.
A conventional method of determining fatigue life of a metal relying upon the X-ray diffraction, consists of sticking a metal foil onto a portion that is to be measured, subjecting the metal foil to the fatigue together with the portion that is to be measured, and irradiating the metal foil with X-rays to determine the degree of fatigue of the portion that is to be measured based upon the width of integration of the resulting X-ray diffraction line. Alternatively the fatigue life is determined from a relation between the change in the residual stress and the change in a half-value width of the diffracted X-rays in the case of a carbon steel that is annealed or in the case of a material that is subjected to the machining by turning.
According to the former method of sticking the metal foil, however, it is difficult to detect the damage by fatigue of the portions where the metal foil cannot be stuck. Furthermore, it is difficult to so stick the metal foil that it is subjected to the fatigue to the same degree as the specimen that is to be measured, and to peel the metal foil from the specimen in such a manner that the damage by fatigue is not changed. The above-mentioned problems do not develop with the latter method by which the specimen is irradiated with the X-rays and an intensity curve of the diffracted X-rays is utilized. With the latter method, however, no change is recognized in the half-value width of the diffracted X-rays depending upon the processing method or the heat treatment, even when the specimen is deformed by fatigue. Namely, the damage by fatigue is not often detected, and the fatigue life is not accurately determined.